FIG. 1 shows the LTE system access flow chart. It encompasses the following processing steps:
Step 102: When the user equipment (UE) needs to connect to the Long Term Evolution (LTE) base station (eNB), it first sends to the eNB the preamble. Since this is the first message of RRC connection procedure, it is referred to as message 1 (i.e. msg1) in the professional jargons.
Step 104: Having detected the preamble, the UE sends back a random access response (referred to as RAR). Since this is the second message in the connection procedure, the entire message transmitted in this step is referred to as message 2.
Step 106: Upon receiving RAR, the UE sends an RRC connection request. As this is the third message in the connection procedure, the entire message transmitted in this step is referred to as message 3. Note that the entire RRC message in the MAC overhead and MAC PDU belong to the content of msg3. It is to be pointed out that the UE, during the entire connection procedure, does not always have to send RRC connection request message. Other messages could be sent in different procedures. For instance, in case of the NBIoT system, UE sends the RRC Suspend/Resume message in message 3, during the newly introduced RRC Suspend/Resume mechanism. Therefore, the RRC connection request is only one of the possible messages that can be carried by msg3.
Step 108: The eNB responds to the RRC connection request by sending back a RRC connection setup message, including signaling radio bearer 1 (SRB1) and contention resolution flag. This is usually referred to as msg4, as it is the message sent in step 4. Since this is the fourth message in the connection procedure, the message sent in this step is referred to message 4.
Step 110: Based on the content of message 4, the UE determines whether its access contention succeeds. If contention succeeds, it sets up the SRB1 according to the information carried by message 4 and sends the RRC connection setup complete message, according to the SRB1 carried by msg4. Since this message is the fourth message in the connection procedure, message sent in this step is usually referred to as message 5, where message 5 contains non-access stratum (NAS) messages such as attach or service request, etc.
Currently, in order to assure a reasonable distribution of radio resource among UEs, LTE system requires that each UE reports the status of the data amount available for transmission stored in its inner buffer. The report is sent to the eNB as the Buffer Status Report (BSR). In LTE system, the Logical Channels (LCH) of a UE are grouped into 5 Logical Channel Groups (LCG). A BSR reports the group sequence number and the information about the data available for transmission in all LCHs. The BSR is transported by the Physical Uplink Shared Channel (referred to as PUSCH)
In LTE system, the time interval for data transmission over the wireless link is referred to as transmission time interval (TTL). Sine BSR is an important reference information for eNB to schedule the UE radio resource, LTE has specified many types and transmission rules for BSR. Depending on the triggering events, BSR can have three types: the regular buffer status report (Regular BSR), the periodic buffer status report (Periodic BSR) or the padding buffer status report (Padding BSR). Here, the regular BSR has the following trigger events:
1. Arrival of upper layer data for transmission on the logical channel that has a higher priority than those currently stored in the logical channel (LCH).
2. Change of the serving cell.
3. Retransmission timer (RETX_BSR_TIMER) in the BSR expires, while data are available for transmission in the UE buffer.
The triggering condition for periodic BSR includes expiration of the periodic BSR timer (PERIODIC BSR TIMER).
The triggering condition for padding BSR includes: When neither Regular BSR nor Periodic BSR waiting for transmission, and the number of padding bits in the assigned resource in the uplink (PUSCH) is greater than, or equal to, the sum of the bits in the control element (CE) of media access control (MAC) and the MAC subheader.
Padding BSR is complimentary to the Regular BSR and Periodic BSR: It has the nature of filling, whereas the Regular BSR and Periodic BSR are non-filling. When no Regular BSR and Periodic BSR are transmitted in the uplink, the Padding BSR can be sent to inform eNB of the LCG data change in the UE buffer more timely.
Regular BSR, Periodic BSR and Padding BSR are transported differently: Regular BSR and Periodic BSR are wrapped in a control element (CE) of the Media Access Control Packet Data Unit (MAC UDP), while the Padding BSR is transported in the Padding bits of MAC PDU, packaged as a MAC CE. The three methods of transporting the BSR differs from each other only in whether the padding bits are used. The MAC PDU is transmitted by PUSCH.
The formats used for transmitting BSR can be further differentiated as short BSR, truncated BSR and long BSR. FIG. 2 shows the 1st format for BSR transmission. FIG. 3 shows the 2nd format for BSR transmission. As FIG. 2 and FIG. 3 show, following the definition of the LTE MAC protocol standard, the format in FIG. 2 is referred to as the short BSR or truncated BSR. The format in FIG. 3 is the long BSR. When a Regular BSR or a Periodic BSR is triggered, and only one LCG has data available for transmission in the TTI, in which the BSR is being prepared for transmission, the UE can choose the short BSR format to transmit the BSR;
When a Regular or Periodic BSR is triggered by UE, while there are more LCG's having data available for transmission in the TTI, in which the BSR is being prepared for transmission, the UE can choose the long BSR format to transmit the BSR. When a Padding BSR is triggered by UE, while there are more LCGs in the TTI in which the BSR is prepared for transmission, and the number of Padding bits in the MAC PDU does not suffice to carry long BSR format and the related MAC subheader, the UE can choose the truncated BSR format for BSR transmission. When the BSR is triggered as Padding BSR by UE and only one LCG with data available for transmission is in the TTI in which the BSR is prepared, the UE can use the short BSR format to report BSR. It is worth noting that the short BSR format and the truncated BSR format have different meanings, even though both use the same format as shown in FIG. 3.
BSR triggering events are all important events. When a Regular BSR is triggered and if no PUSCH resource for transmitting the BSR is availbe in the current TTI, the UE needs to trigger the Scheduling Request (SR), which can be canceled later, if UE gets PUSCH resource in the follow-up TTI. Of course, if there is no PUSCH resource in the follow-up TTI, the SR will be sent to eNB through the Physical Uplink Control Channel (PUCCH), so that the eNB can assign PUSCH resource to the UE.
According to the definition of the current LTE MAC layer protocol standard (e.g. TS 36.321), the BSR is triggered and transmitted as following:
According to the triggering conditions described before, UE determines whether to trigger the BSR in every TTI.
At every TTI, the UE determines whether there is already a triggered BSR. If there is a triggered BSR, the UE needs to determine whether there is PUSCH resource available in the current TTI. If there is available PUSCH resource, the UE selects the appropriate format to construct the MAC CE for the BSR. If there is no BSR triggered, the UE needs to determine whether to trigger a Padding BSR. If triggering a Padding BSR is possible, it needs to select the appropriate BSR format to construct the MAC CE for the BSR. When the MAC CE is completed, the UE executes the uplink transmission.
Power headroom report (PHR) refers to the procedure, when UE uses the method of MAC CE to report the difference between the nominal maximum transmit power and the estimated transmit power of the Uplink Shared Channel (UL-SCH) to the eNB. Conditions for triggering PHR can include the following:
1. The prohibitPHR-Timer expires and the change in path loss is greater than the configured value (computed from the last PHR epoch)
2. The periodicPHR-Timer expires.
3. PHR function entity is configured or reconfigured.
Once the PHR is triggered, the UE transmit the PHR when it has the uplink resource to support PHR. FIG. 4 shows the MAC CE format for PHR. As shown in FIG. 4, the power headroom (PH) is expressed in bits with a length of 6 bits. In addition, there are two reserved bits (R), for which the default value is 0 in the related technical specification.
In LTE system, format type 1 is usually used for power headroom report, where the power headroom value is read from the physical layer in 64 levels.
In current LTE sytems, in order to support machine type communication terminals (e.g. sensor, smart home, intelligent grid etc.), narrow band Internet of things (NB-IoT) is introduced. The system has a bandwidth 180 kHz for the use of machine type communication of small data amount, to avoid the impact of the small data amount on the spectrum efficiency of the terminals designed for high data rate, and, at the same time, to increase the number of users carried by unit frequency bandwidth.
Nevertheless, although the deployment of narrow band system can isolate the machine type terminals and non-machine type terminals, it does not help improving the transmission rate of the user. This is because the narrow band system uses the same control plane and user plane as the wide band LTE, hence the cost for the control is the same for both. Therefore, compared to the LTE system, the narrow band system has no obvious advantage in terms of spectrum efficiency.
In order to improve the spectrum efficiency of narrow band systems and to reduce the overhead of signaling, a concept of transmitting data by NAS signaling is introduced recently to the NB-IoT system. Transmitting data by control plane signaling is an abnormal approach, as the quality of service (QoS) of the signaling transmission is unique, while the data transmission should be able to provide a multitude of QoSs. Therefore, using the signaling transmission mechanism to transmit data would have negative impact on the MAC layer with various consequences.
To summarize, none of the currently known techniques, such as MAC scheduling and HARQ, etc. can cooperate with the concept of transmitting data over signaling channels effectively.